This invention relates to cooling systems and to thermal storage systems which use ice storage.
The object of "cool[ storage (also called "thermal storage") is to extract heat from a thermal reservoir during one time period and, during a different time period, to use the reservoir to extract heat from another environment. One important application of thermal storage is in building air conditioning systems. Ice and chilled water are the usual media for thermal storage. Each has advantages and disadvantages. For example, pure chilled water systems (no ice storage) can use higher refrigeration temperatures than ice storage systems (approx. 30.degree. F. vs. 20.degree. F.). In addition, ice systems have a distinct size advantage, as a rule-of-thumb requiring about one-fifth the storage volume of pure chilled water systems. Because of such desirable features, the ice-based thermal storage systems are experiencing an increasingly wide range and large volume of usage. It is to such ice building systems that the present invention is primarily directed.
Ice-based thermal storage systems can be classified as dynamic or static. In dynamic systems, ice is made in chunks or as crushed ice and is stored in large containers. In static systems, ice is formed on the cooling coils of the storage vessel itself. FIG. 1 is a block diagram example of an application system 10 which uses a prior art ice-based static thermal storage system. Static ice-storage system 10 utilizes an open tank or other unpressurized water heat exchanger 11, with heat extraction being provided, for example, by piping refrigerant through evaporator tubes in the water. It should be noted that, as used herein, "unpressurized" means open or at atmospheric pressure, whereas "pressurized" means above atmospheric pressure at the surface of the water.
The system 10 is typical of prior art application systems in that it comprises three major component systems or subsystems: a pressurized refrigeration system; an unpressurized chilled water heat sink system; and a pressurized chilled water utilization system which includes the building air conditioning loads. The refrigeration system includes the evaporator section of unpressurized heat exchanger system 11, a compressor 13 which withdraws and compresses the gaseous refrigerant from the heat exchanger, and a condensing heat exchanger 14 for cooling and condensing the refrigerant gas to liquid before it is returned to the evaporator in heat exchanger system 11.
The pressurized chilled water utilization system includes a second heat exchanger system 16, which includes a pressurized component. A pump 17 circulates system water between the two heat exchanger systems to extract heat from the chilled water utilization system at heat exchanger 16 and in turn have heat extracted by the refrigeration based ice storage system at heat exchanger 11. Finally, the cool water utilization system includes a pump or pump system 18 for circulating the water through the pressurized component in heat exchanger system 16 and other heat exchange water coils included in the system.
The cooling/air conditioning system 10 has achieved increasingly wide-spread application during the past several years, in no small part due to the fact that it is the only previously known application technology for such ice-storage water-circulation systems. A strong impetus for use of thermal storage for commercial building air conditioning systems has arisen from the difficulty that commercial power companies have experienced in bringing up a sufficient electrical power generation capability to handle peak electricity usage, especially in major metropolitan areas having widespread use of commercial air conditioning. The peak power demand on the power generation capability of the utility on a very hot day can put a severe strain on the power generation system. About thirty percent of the summer peak load is contributed by commercial air conditioning demand. This contrasts with an approximate two percent contribution by residential peak cooling demand. Thermal storage is the only approach that can shift electricity usage from a peak demand period to an off-peak period. Thus, it is anticipated that thermal storage for commercial air conditioning systems and other chill water system applications will become increasing important in the future.
Moreover, it has been shown that with proper application of thermal storage technology, even utilizing prior art approaches, an ice storage system can be competitive with a conventional centrifugal cooling plant in a commercial air conditioning system. A case history of such an implementation of a prior art type of thermal storage system using ice building technology of the prior art is set forth in a paper by Gilbertson and Jandu entitled "Twenty Four-Story Office Tower Air-Conditioning System Employing Ice Storage--A Case History." This paper was presented at an ASHRAE conference in Atlanta, Ga. in January, 1984, and will later be published in the "Transactions" of that conference and the discussion therein is hereby incorporated by reference.
As noted in the Gilbertson et al. paper, thermal storage systems of the ice building type provide a number of advantages over conventional centrifugal cooling plant systems. The major advantages are lower operating costs, reduction in certain building costs, improvements in reliability, and reduction in maintenance. Furthermore, in some instances thermal storage of the ice building type will enable commercial air conditioning to be implemented in extremely hot climates in which conventional centrifugal cooling plants are essentially useless during peak day time temperatures. In addition, thermal storage of the ice building variety may enable the benefits of commercial air conditioning to be utilized in developing countries which have limited power generation capacity. Shifting commercial air conditioning load requirements to cooler night time hours reduces the need for new power plants and, in addition, provides more steady, efficient usage of existing power stations by reducing load shifting and starting and stopping of generation equipment.
The major contribution to lower operating costs of an ice storage system is the availability of less expensive electricity at night to store cooling capacity which is then available to meet peak air conditioning loads during the day. Electricity utilization is also more efficient during night time hours when temperatures are lower and heat rejection to the ambient atmosphere is more efficient. In certain parts of the United States, total savings of sixty percent on electricity costs are achievable.
However, prior art thermal storage systems also have limitations and undesirable features. For example, the bulk and weight of the water and vessel of system 11 used in such applications dictates that it be placed at ground or grade level in all but the smallest applications. However, where all or part of the load/utilization system is higher than the heat exchanger, constraints imposed by the water head of the utilization system and pumping requirements prohibit the use of an unpressurized utilization system. That is, the second heat exchanger system 16 is required to interface the pressurized and unpressurized systems. The use of an open atmosphere heat exchanger 11 and associated system and the pressurized heat exchanger 16 and associated system requires considerable investment in apparatus and interconnections. The requirement of an additional heat exchanger system 16 interconnecting the open tank and sealed chill water systems also reduces the heat transfer efficiency of the system.
In addition, while open atmosphere water tanks are euphemistically referred to as "sweet water" systems, they are anything but sweet. The large volume of water is subject to contamination by the external environment. The system components are subject to rust and to deterioration caused by alternate wetting and drying as the water level changes due to changes in the volume of the hydraulic system resulting from the freezing and thawing of ice.
It is thus apparent that there is a need for a better approach to thermal storage systems.